Tyzzer's disease has been reported in a wide range of laboratory, domestic, and wild animals. Acute infection is characterized by a fatal hemorrhagic enteritis, focal necroses in the liver, and the presence of many intracellular bacilli surrounding the lesions. Acute disease appears to be rare in natural infections, but seemingly healthy animals can harbor a latent form of infection which can develop into acute disease when the animal is subjected to stress. An outbreak of the disease among laboratory animals during the course of an experiment is costly both scientifically and economically. The effects of latent disease on research utilizing infected animals, although potentially significant are not known. Studies of the unclassified etiologic agent, Bacillus piliformis, have been hampered by the difficulties in culturing this obligate intracellular pathogen. The objectives of this proposal include: 1)Biochemical, immunological, and genetic characterization of the causative bacteria to determine the taxonomical classification of the organism and the relatedness of B. piliformis isolates from different host species. 2) Development of improved diagnostic assays using cloned genes and gene products to afford more sensitive, specific, and earlier detection of the disease. 3)Examination of the host:parasite interactions that permit intracellular survival of the pathogen and elucidation of the host response to clinical and asymptomatic disease. For the first time, the experiments designed in this proposal will utilize molecular techniques to examine and probe the causative agent of disease at the genetic level. Moreover, cloning and expression of B. piliformis genes in Escherichia coli will circumvent the limited availability of pathogen components due to the difficulties in culturing the obligate intracellular parasite. These components are needed for the biochemical characterization of the organism and its pathogenesis. The results of these studies will contribute to the basic understanding of Tyzzer's disease, and may potentially yield solutions to the problem of control and prevention of the disease among laboratory animals.